Nanoporous Titanium (Oxy)nitride Films as Broadband Solar Absorbers

Broadband absorption of solar light is a key aspect in many applications that involve an efficient conversion of solar energy to heat. Titanium nitride (TiN)-based materials, in the form of periodic arrays of nanostructures or multilayers, can promote significant heat generation upon illumination thanks to their efficient light absorption and refractory character. In this work, pulsed laser deposition was chosen as a synthesis technique to shift metallic bulk-like TiN to nanoparticle-assembled hierarchical oxynitride (TiOxNy) films by increasing the background gas deposition pressure. The nanoporous hierarchical films exhibit a tree-like morphology, a strong broadband solar absorption (∼90% from the UV to the near-infrared range), and could generate temperatures of ∼475 °C under moderate light concentration (17 Suns). The high heat generation achieved by treelike films is ascribed to their porous morphology, nanocrystalline structure, and oxynitride composition, which overall contribute to a superior light trapping and dissipation to heat. These properties pave the way for the implementation of such films as solar absorber structures.


S2
This file includes: Figure S1. EDX microanalysis of the TiN target. Figure S2. High-resolution XPS spectra for the film deposited at 100 Pa. Figure S3. Detailed optical spectroscopy results. Figure S4. Ellipsometry measurements. Figure S5. Details on the thermal camera measurements. Figure S6. Top view SEM images of an uncoated Ti substrate and of all the investigated films on Ti substrates. Figure S7. Temperature profiles under solar irradiation. Figure S8. Raman spectra before and after solar irradiation for all the investigated films.

Supplemental Figures
Supplemental Tables   Table S1. Quantitative chemical analysis by XPS. Table S2. Details on XPS peak fitting for the film deposited at 100 Pa.

Supplemental Notes
Note S1. Details on the interpretation of Raman spectra. Note S2. Details on optical spectroscopy measurements. Note S3. Details on ellipsometry measurements. Figure S1. EDX microanalysis of the TiN target. The quantitative reliability of EDX measurements is affected by the small atomic number of nitrogen and oxygen and the vicinity in energy of their x-ray transitions with the lower one of titanium (i.e., K = 0.392 keV and 0.525 keV for N and O atoms, respectively; K = 4.512 keV and L = 0.452 keV for Ti atom). To account for such effects and to ensure reliability of the atomic composition in the films, the nitrogen to titanium ratio of the stoichiometric TiN target was measured and the value N/Ti = 0.8 was found. Moreover, the target also exhibited the presence of oxygen (13% at.), associated to a thin native oxide overlayer which forms on TiN surface because of air exposure. 1 S4 Figure S2. High-resolution XPS spectra for the film deposited at 100 Pa: (a) O 1s, (b) Ti 2p, (c) N 1s, and (d) C 1s regions. The spectra suggest a high oxidation degree of the surface, consistent with a TiOxNy material (see also Table S2). The relatively high amount of adventitious carbon (d)

Supplemental Figures
can be ascribed to the high porosity of the film.  Table S1). Further details on the ellipsometry measurements are reported in Note S3. It should be noted that the results for the films deposited at 50 and 100 Pa were affected by their low reflectance ( Figure S3c) and high haze factor ( Figure   S3e), which limits the signal collection by the detector in the ellipsometer (which works in specular reflection mode with high angles of incidence, see Note S3). S10 Figure S8. Raman spectra before and after solar irradiation for all the investigated films. Raman spectra after irradiation were acquired on films that were tested for temperature measurements from 1.3 to 17 Suns ( Figure S7).

Supplemental Notes
Note S1. Details on the interpretation of Raman spectra.
In principle, first-order Raman scattering is not allowed in an ideal crystal with rock-salt cubic structure like TiN (i.e., Oh symmetry). However, films deposited by means of physical vapor deposition present defects that may be generated from energetic ions or species in deposition mechanism, leading to a reduction of crystal symmetry which induces first-order Raman modes composed by broad bands. In particular, the relative intensity of acoustic and optical bands can be used as an indication of film stoichiometry. Indeed, acoustic bands occurring at ~ 200-215 cm -1 and ~ 300-330 cm -1 are associated to vibrations of Ti 4+ ions due to nitrogen vacancies, while the optical mode at 500-600 cm -1 is associated to Ti vacancies and vibration of N 3ions. 7 Therefore, a high Iacoustic/Ioptical ratio (>1) is related to a high number of nitrogen vacancies and, therefore, to under-stoichiometric TiNx (with x < 1), Iacoustic/Ioptical ~1 is associated to nearly-stoichiometric TiN, while Iacoustic/Ioptical <1 over-stoichiometric TiNx (with x > 1). 8 S14 Note S2. Details on optical spectroscopy measurements.
Optical spectroscopy measurements in the ultraviolet-visible-near infrared (UV-vis-NIR) range were performed in transmittance and reflectance mode on samples grown on glass substrates. The absorptance was calculated according to the formula is the total transmittance, given by the sum of the specular and the diffuse components, and is the total reflectance, also given as the sum between the specular and diffuse components. The haze factor, on the other hand, was evaluated according to the formula 9 The measurement of the total components for both the transmittance and the reflectance was ensured by the use of an integrating sphere in the UV-vis-NIR spectrophotometer. Therefore, Figures S3a, S3c and S3e show the total transmittance, total reflectance and the haze factor, respectively, while Figure 5a in the main text shows the absorptance calculated according to Equation (S1).
Optical spectroscopy measurements in the medium-infrared (MIR) range were instead performed in transmittance and reflectance mode on samples grown on silicon substrates with a FTIR spectrometer, using a thick gold film as a calibration reference for the reflectance and pumping the chamber to vacuum prior to the measurements. The FTIR spectrometer was not equipped with an integrating sphere; therefore, Figures S3b and S3d show the specular transmittance and reflectance, respectively. As a consequence, Figure 5c in the main text shows the absorptance calculated as ,,